The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of a rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Sensors can also be categorized as current in plane (CIP) sensors or as current perpendicular to plane (CPP) sensors. In a CIP sensor, current flows from one side of the sensor to the other side parallel to the planes of the materials making up the sensor. Conversely, in a CPP sensor the sense current flows from the top of the sensor to the bottom of the sensor perpendicular to the plane of the layers of material making up the sensor. In a CPP sensor design, the magnetic shields usually double as electrical leads for supplying a sense current to the sensor. Therefore, in CPP sensor design, the shields/leads contact the top and bottom of the sensor.
The ever increasing demand for data storage density and data rate have increasingly pushed the limits of data storage designs. Recently in efforts to overcome such limits, engineers and scientists have focused on the use of perpendicular recording. In a perpendicular recording system a write pole emits a highly concentrated magnetic field that is directed perpendicular to the surface of the medium (eg. the disk). This field in turn magnetizes a localized portion of the disk in a direction perpendicular to the surface of the disk, thereby creating a bit of data. The resulting flux travels through the disk to a return path having a much larger area than the area in which the bit was recorded. The increased interest in perpendicular recording has lead to an increased interest in current perpendicular to plane (CPP) sensors, which are particularly suited to use in perpendicular recording.
The ever increasing demands for increase data density and data rate have also led engineers to look at the possibility of constructing dual GMR sensors. Such a sensor would have two free layer/pinned layer interfaces, such as by having a free layer sandwiched between two pinned layers. However, many challenges presented by the use of such a design have prevented dual GMR sensors from being used, and to date no commercial dual GMR sensor has been produced. One problem presented by such a design is that the use of a dual GMR sensor takes up too much gap budget (the thickness of the sensor as measured from one shield to the other). Increased sensor thickness leads to increased bit length, and therefore leads to decreased data density and data rate. A significant contributor to this sensor gap increase has been due to the fact that each pinned layer requires its own antiferromagnetic AFM pinning layer. AFM layers used to pin the magnetic moment of a pinned layer through exchange coupling are very thick, often being significantly thicker than all of the other sensor layers combined. In addition, in a CPP sensor the AFM layer adds additional sheet resisistance (the denominator in the dr/R measurement), thereby decreasing the sensitivity of the sensor. Therefore, the use of multiple AFM layers becomes impractical.
In addition, the use of AFM layers increases to the distance between the spacer layers of each of the pinned layer/spacer layer/free layer interfaces. This causes each interface to read a different part of the magnetic signal, which tends to be a generally bell shaped curve. Reading different parts of signal curve leads to poor signal reading and increased noise, rendering such sensors impractical.
Therefore, there remains a need for a practical sensor that can provide improved dr/R GMR performance needed for future generation memory storage products. Such a sensor would preferably be usable as a current perpendicular to plane sensor, since these are the sensor that will be most attractive for use in future perpendicular recording systems. Such a sensor would preferably minimize the number of AFM pinning layers while maximizing the number of free layer/spacer layer/pinned layer interfaces in order to maximize the dr/R of the sensor. Such a sensor would also preferably minimize the spacing between each of the free layer/spacer layer/pinned layer interfaces so that the sensor will read, as much as possible, only a single portion of the signal curve.